This application, in response to RFA AI-94029 is a collaboration to define the mechanisms and pathogenesis of perinatal transmission and infection in a cohort of 350 pregnant HIV seropositive women and their children followed at the University of Zimbabwe from 1991-1995. The perinatal transmission study in Zimbabwe, sponsored by the WHO has created a sample bank of serial blood samples from infants from birth through 2 years, blood and breast-milk specimens from their mothers and clinical and serologic follow-up of 700 women (50% of whom were HIV seropositive at enrollment) and their infants for two years. The studies proposed is focused on polymerase chain reaction (PCR) amplification of proviral DNA and virion RNA from infant samples to determine the frequency and timing of transmission as well as the pathogenesis of infection in the context of early and late vertical transmission. Based on an estimated overall frequency of infection in infants of 40%, more than 100 transmission positive mother baby pairs will be identified and compared to 200 non- transmitting mothers and their infants. Quantitative DNA and RNA PCR will be used to characterize virus load in mothers and their infants to define differences in transmission as well as the pathogenesis of infection in the context of early and late vertical transmission. The universal practice of breast-feeding in this cohort and the collection of breast milk at each post-natal visit provides samples and data to investigate the frequency and risk of transmission by this route. To define the virologic risk factors for transmission, molecular methods including envelope gene sequencing and heteroduplex gel mobility analysis and homoduplex tracking assays to determine quasispecies diversity will be used to define the specific HIV envelope characteristics of viruses in mother-infant pairs. In the same samples, serologic markers for protection from (or facilitation of) vertical transmission will be sought by measuring neutralizing binding and enhancing antibodies against contemporary and homologous virus isolates in maternal serum and breast milk. Current studies in Zimbabwe indicate that HIV infection is caused by Clade C viruses with the GPGQ V3 loop-crown. The planned studies in Zimbabwe will parallel virologic and serologic studies underway in maternal-infant studies (of Glade B viruses) from the U.S. and Europe. In addition, breast feeding in Zimbabwean cohort adds another dimension to vertical transmission that is applicable to much of Africa and the developing world. Because perinatal and sexual transmission of Clade C HIV will result in millions of cases of AIDS in Africa and Asia, understanding the mechanisms and pathogenesis in this viral genetic context is important in the development of vaccines and other intervention strategies to prevent vertical transmission of HIV.